The invention relates to a color picture display device comprising a cathode ray tube, a means for generating three electron beams, a display screen and a deflection unit for generating deflection fields for deflecting the electron beams across the display screen, provided with compensation means which extend in front of the deflection plane of the deflection unit and are used for compensating picture errors.
The invention also relates to a deflection unit for a cathode ray tube.
A picture display device of the type described in the opening paragraph and a deflection unit of the type described in the second paragraph are known from European patent specification EP 0 187 964. On its side facing the display screen, the known deflection unit is provided with a system of permanent magnets so as to compensate picture errors. The system comprises four main bar magnets and eight smaller adjustable magnets. The adjustable magnets may be rotated for the purpose of fine-tuning.
The drawback of the known picture display device and deflection unit is that the design is complicated and the adjustment of the fine-tuning magnets is a time-consuming process.
It is an object of the invention to provide a picture display device allowing a simplified design of the picture display device and deflection unit.
To this end, the picture display device according to the invention is characterized in that the compensation means comprise four magnet systems which, by approximation, are arranged fourfold symmetrically with respect to the tube axis, and in which each magnet system, viewed from the tube axis, extends through an angle ranging between 24 and 34 degrees or between 40 and 48 degrees.
The invention is based on the recognition that magnet systems placed in front of the deflection plane, for example, on the side of the deflection unit facing the display screen, may have a positive effect on picture errors, but may generally introduce picture errors in their turn.
Fourfold symmetrical is herein understood to mean that the magnetic fields of each magnet system are approximately equal, be it that the magnetic fields, when taking the tube axis (or the axis of symmetry of the deflection unit) as a reference, differ in strength and direction to such an extent that they merge with each other around said axis due to a rotation through an angle of 90 degrees. By using four magnet systems which, by approximation, are arranged fourfold symmetrically with respect to the tube axis (or axis of symmetry of the deflection unit), the following aspect occurs.
Due to the symmetrical arrangement, the common magnetic field generated by the magnet systems (and hence the field generated by the compensation means) can comprise only 4, 8, 12, 16 poles, etc. The four magnet systems comprise two pairs of magnet systems, for example, an east-west pair and a north-south pair. The 4-pole, 12-pole and 20-pole fields generated by each pair have opposite signs and cancel each other. As a result, the magnet field generated by the compensation means comprises only 8-pole, 16-pole, 24-pole, etc. components. The compensation means notably generate a magnetic field which, in addition to a desired 8-pole with which picture errors can be compensated, comprises also higher-order components. These higher-order components, notably 16 and 24-pole components, cause unwanted picture errors. These higher-order components in the magnetic field increase the sensitivity to tolerances in dimensioning and positioning and residual errors such as variations in the triorotation error across the display screen.
The consequences of the unwanted higher-order components in the magnetic field generated by the magnet systems may be alleviated by placing the magnet systems further away from the tube axis. However, this also results in the necessity of using stronger magnet systems so as to achieve the desired positive effect, which increases costs, may affect proximate further elements of the color display device and increases the size of the deflection unit.
The consequences of the unwanted higher-order effects may also be alleviated by using fine-tuning magnets as described in the European patent specification cited hereinbefore. However, these fine-tuning magnets are expensive, complicate the design, must be adjustable and, moreover, should be fixed after their adjustment.
The invention provides an improved picture display device of the type described in the opening paragraph.
The invention is based on the recognition that the strengths of higher-order components in the magnetic field generated by the magnet systems can be minimized by suitable choice of the design of the magnet systems so that the negative effects caused by the magnet systems are reduced without reducing the strength of the desired component and hence the positive effect of the magnet systems.
Each higher-order component has a design for which the strength of the higher-order component is minimal. The strength of the 16-pole component is theoretically minimal for four magnet systems each extending through an angle of 45 degrees, the strength of the 24-pole component is theoretically minimal at 30 degrees. Generally, the strength of the 16-pole component is very small if the magnet systems extend through an angle ranging between 40 and 48 degrees. The strength of the 24-pole component is very small if the magnet systems extend through an angle ranging between 24 and 34 degrees.
The magnet systems preferably extend through an angle ranging between 24 and 34 degrees, preferably between 28 and 30 degrees. The component having the highest order is then minimized and the length of the magnet systems is shorter than in the range between 40 and 48 degrees, which reduces the risk of the magnet systems affecting other elements of the deflection unit and/or color display device, such as the IMS (Internal Magnetic Shield).
Within the scope of the invention, the magnet systems may be formed by electromagnets each comprising, for example, a coil and an elongate core, but the compensation means preferably comprise four permanent magnets of equal size and strength.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.